The precise measurement of flow velocities, flow volumes, and other properties which affect the electrical conductivity is of great importance, for example, for metallurgical processes, for the growing of semiconductor monocrystals, and for the production of glass. These substances are hot and corrosive during the melt process, so that contactless electromagnetic methods are especially suited for such measuring chores.
Contactless electromagnetic flow measurement methods are known from the publications DE 33 47 190 A1, DE 43 16 344 A1, DE 199 22 311 C2, DE 100 26 052 B4 and the publication “The use of magnetohydrodynamic effects to investigate fluid flow in electrically conducting melts” (J. Baumgartl, A. Hubert, G. Müller, Physics of Fluids, Vol. A 5, Series 1993, pages 3280-3289), in which a magnetic field (the so-called primary field) is coupled into the substance and the magnetic field disturbance induced by eddy currents (the so-called secondary field) serves as a measure of the flow velocity. These methods can be used to determine both mean flow velocities and spatial distributions of the flow velocity, in the latter case making use of the method of least squares of the error to determine the flow distribution.
However, these already known measurement methods have three serious drawbacks. First, due to their limited sensitivity, they do not allow measuring flow velocities in very slowly flowing or very weak electrically conductive substances, such as glass melts. Second, the measurement accuracy of the systems is greatly limited in an electromagnetically perturbed environment, since the magnetic field sensors will be affected by even the slightest parasitic fluctuations of the magnetic field, due to their small spatial dimension. Third, the measurement sensitivity of the method, being characterized by the ratio between secondary field and primary field, cannot be increased by heightening the primary field.
The mentioned drawbacks are only partly remedied by the methods as described in the publications JP 571 999 17 A, U.S. Pat. No. 6,538,433 B1 and JP 071 811 95 A. These are characterized in that, rather than make a direct measurement of the secondary field, one performs a measurement of the force exerted by the secondary field on the system producing the magnetic field. Despite their advantages over the first mentioned methods, the force-measuring systems are of little or no use for a number of important special problems, including flow measurement.
In the layout portrayed in JP 571 999 17 A, the primary magnetic field is generated by coils through which currents flow, entirely surrounding the tubular flow being measured. Such a system is very heavy, it requires an elaborate current supply, and can only be transported to a different place of use after costly disassembly of the measurement system.
The systems presented in the documents U.S. Pat. No. 6,538,433 B1 and JP 071 811 95 A involve local sensors, which can only measure the flow velocity in their immediate surroundings. These local sensors are not suited to determining the mass flow or the volume flow, because the magnetic field generated by them only penetrates part of the cross section through which the conductive substance is flowing. Furthermore, the measurement sensitivity of the local sensors declines with the third power (or even the fourth in the case of greater distances) of the distance from the conductive substance and is therefore not adequate for many application instances.